1. Field of the Invention
The present invention relates to a method and a device for displaying images on an array screen by activation of screen pixels arranged in rows and columns.
2. Discussion of the Related Art
The present invention especially applies to array screens in which each pixel is formed of a light-emitting diode, for example, of organic or polymer nature (screen of OLED type, for Organic Light-Emitting Display or of PLED type, for Polymer Light-Emitting Display). The cathodes of the diodes of a same row are connected to a row electrode and the anodes of the diodes of a same column are connected to a column electrode.
FIG. 1 shows an example of a conventional device for displaying an image on an array screen.
In the present example, the image is displayed on a screen 10 in the form of a single frame. Each pixel 12 is then associated with a memory point 14 of a frame memory 16, for example, a RAM. Memory points 14 are arranged in Y rows and in X columns and may be set to state 0 or to state 1 via a writing interface 18 which receives data to be written WDATA as well as addresses WADDRESS of these data. Writing interface 18 is controlled by a write clock signal WCLK. The image to be displayed on screen 10 is previously stored in digital form in frame memory 16. As an example, a memory point 14 is set to 1 when the corresponding pixel 12 of screen 10 is to be on, and set to 0 otherwise.
Upon normal operation of the display device, the image stored in frame memory 16 is displayed on screen 10 at the frequency of a frame clock signal FCLK. The states of all the memory points 14 of a row may be read by a reading interface 20. Reading interface 20 receives at the frequency of a read clock signal RCLK an address signal RADDRESS provided by an address counter 22 which indicates the row of frame memory 16 to be read. Reading interface 20 then provides a column driver 23 with data RDATA, for example in the form of X bits, which represent the states of memory points 14 of address row RADDRESS. A row driver 24 also receives an address signal ADDRESS, for example, in the form of Y bits, corresponding to the image of address RADDRESS by a decoder 26 and which enables activation of the row of screen 10 associated with address row RADDRESS of frame memory 16. Column driver 23 then activates or not the pixels 12 of the activated line according to data RDATA. As an example, the pixels activated in FIG. 1 are shown by crosses 27.
In certain applications, especially for portable phone or electronic diary screens, the image stored in frame memory 16 may not be modified upon display of many successive frames. Certain pixels 12 of screen 10 are then activated and others are off for many successive frames. This result in an uneven aging of pixels 12 of screen 10.
To avoid uneven aging of the screen pixels, different successive images are attempted to be displayed on screen 10 even if the image stored in frame memory 16 is fixed or varies sporadically for several successive displayed frames.
FIG. 2 shows a device for displaying images on an array screen which provides a specific operating mode, which enables displaying different successive images on the screen based on an image stored in a stand-by memory 30 which is fixed or varies sporadically.
A CPU 32, driven by the microprocessor of the display device, is adapted to reading the states of the memory points of stand-by memory 30, to determining a new image by calculating new states, and to writing the new states in frame memory 16 via writing interface 18. Based on the image stored in stand-by memory 30, CPU 32 thus determines a new image stored in frame memory 16 which will be usually read by reading interface 20 and displayed on screen 10 as has been explained previously.
In stand-by mode, the new image calculated by CPU 32 often corresponds to the image stored in stand-by memory 30 offset according to the directions of the rows and columns. According to the frequency at which CPU 32 provides new images stored in frame memory 16, a spectator can sense the image of frame memory 30 which displaces on screen 10.
A disadvantage of such a device is that the CPU must generate new images at a sufficient frequency with respect to the frequency of frame clock signal FCLK to obtain a satisfactory motion of screen 10. Thereby, the work capacities of the device microprocessor are required for the calculation of new images, which increases the work load of the microprocessor.